1. Field of the Invention
The present invention relates to LED brightness control systems and more particularly to the control of an LED source for LCD backlighting over a wide range.
2. Description of the Related Art
In the manufacture and use of liquid crystal displays, backlighting from a light source is dispersed evenly beneath the surface of a liquid crystal display to allow optimal viewing of the display in all types of ambient light conditions. Depending upon the light conditions of the environment, the light intensity of the backlight may be adjusted to allow optimal viewing of the liquid crystal display. Liquid crystal display backlights frequently employ fluorescent lamps. However, fluorescent lamps require high power and a high voltage source. Another method of providing backlight is through the utilization of light emitting diodes. Light emitting diodes are utilized for backlights in liquid crystal displays due to their dimming range, low-temperature performance, and efficient heatsinking attributes.
The brightness of an light-emitting diode (LED) source for Head-Up Display (HUD) liquid-crystal display (LCD) backlighting must be controlled over a range of at least 20,000 to 1. It is impractical to drive the LED illuminator, which can consist of an array of LEDs, with a variable DC source to very low current levels as the LED light output becomes unstable with possible spectral shifts.
There are a number of patents that discuss different brightness controls for LED sources for LCD backlighting. For example, U.S. Pat. No. 6,608,614 B1, issued to R. J. Johnson, discloses a backlight for an LCD that includes a first LED array that provides light with a first chromaticity and a second LED array that provides light with a second chromaticity. A combining element combines the light from the first LED array and the second LED array and directs the combined light toward the liquid crystal display. A control system is operationally connected to the second LED array. The controller adjusts the brightness of at least one LED in the second LED array to thereby adjust the chromaticity of the combined light.
U.S. Pat. No. 6,392,358, issued to A. L. Runau, discloses a system and method for directing the flow of current supplied for a collection of light emitting diodes to provide for rapid flashing of the light emitting diodes and a wide dimming range. A fixed amount of current may be available to the collection of light emitting diodes where the flow of the current may be controlled by an alternating periodic signal such that current passes through the light emitting diodes for predefined and discrete periods. During periods of time that current is not flowing through light emitting diodes, the current may be directed to flow in another area of the circuit. The duty cycle of the signal may be adjusted in order to vary the dimming capability of the backlight.
U.S. Pat. No. 5,803,579, issued to R. R. Turbull et al, disclose an illuminator assembly, having a plurality of LEDs on a vehicular support member in a manner such that, when all of the LEDs are energized, illumination exhibiting a first perceived hue, e.g., blue-green, and projected from at least one of the LEDs overlaps and mixes with illumination exhibiting a second perceived hue, e.g., amber, which is distinct from the first perceived hue and which is projected from at least one of the remaining LEDs in such a manner that this overlapped and mixed illumination forms a metameric white color and has sufficient intensity and color rendering qualities to be an effective illuminator.
Generally, the above patents do not address controlling LED brightness over a wide range. The '358 patent discusses using PWM (pulse width modulation) with a fixed current level. As will be disclosed below, the present patent application provides PWM control in addition to variable current control, to provide a much wider range of brightness control by performing a “multiplying” function to the two control inputs (peak current control and PWM control).